Field of the Invention
The present invention relates to a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing pad (polishing surface), and more particularly to a substrate holding apparatus for holding a substrate such as a semiconductor wafer in a polishing apparatus for polishing and planarizing the substrate. Further, the present invention relates to a polishing apparatus having such substrate holding apparatus.
Description of the Related Art
In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). In the chemical mechanical polishing, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface of a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface and polished by using the polishing apparatus.
The polishing apparatus which performs the above-mentioned CMP process includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding apparatus, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When the substrate is polished with such a polishing apparatus, the substrate is held and pressed against the polishing surface of the polishing pad under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relative to each other to bring the substrate into sliding contact with the polishing surface, so that the surface of the substrate is polished to a flat mirror finish.
If a relative pressing force produced between the substrate being polished and the polishing surface of the polishing pad is not uniform over the entire surface of the substrate, then the substrate is insufficiently or excessively polished depending on the pressing force applied to each area of the substrate. Therefore, it has been attempted that a substrate-holding surface of the substrate holding apparatus is formed by an elastic membrane made of an elastic material such as rubber, and a plurality of pressure chambers to which a pressurized fluid is supplied are formed at the reverse side of the elastic membrane and a fluid pressure such as air pressure is applied to the pressure chambers to uniform the pressing force applied to the substrate over the entire surface of the substrate.
In the above polishing apparatus, when the substrates are polished using a polishing pad made of synthetic resin, the polishing pad is progressively worn each time it is dressed and with the passage of polishing time. In order to keep the surface pressure distribution unchanged on the substrate held by the top ring, it is necessary to keep the distance between the top ring and the polishing pad constant during polishing.
When a product substrate is processed, the top ring is moved by a servomotor to a contact position (height), between a substrate held by the top ring and the polishing pad, determined beforehand by an operation referred to as pad search which is a function or action for determining a reference height position for polishing, and the product substrate is polished at the contact position (height) in a positioning control state. In some cases, the top ring is lifted from the contact position (height) to a certain height corresponding to a clearance between the polishing pad and the membrane, and the product substrate is polished in a positioning control state.
In the operation of pad search for determining the above contact position, for example, if the contact position is determined by simply measuring a distance to the polishing pad by a distance measuring device or the like, it may cause considerable error because the polishing pad is made of an elastic material and has a concavo-convex surface. Therefore, it has been customary that the top ring is lowered from a lifted position onto the surface of the polishing pad to detect a contact force against the polishing pad, thereby determining the contact position. The contact force is detected by monitoring an output torque (output current) of the servomotor of a positioning mechanism for lifting and lowering the top ring.
The function or action referred to as the pad search is a function for determining the contact position (height) between the substrate held by the top ring and the polishing pad from a certain reference height (for example, a transfer height in a horizontal direction) of the top ring. When performing this function, a torque limit value which has been obtained beforehand is set, and the top ring is stopped at the position at which the top ring is brought into contact with the polishing pad and the output torque of the servomotor becomes the preset torque limit value, and then this position is set as a reference position (height) for polishing.
The pad search operation will be described further in detail below. The torque limit value for the pad search is determined beforehand. The method for determining the torque limit value is as follows: For example, first, an initial value of the torque limit value is set from a design value. Then, a load cell is placed on the polishing table to which the polishing pad is attached, and the top ring is lowered until the set torque limit value (initial value) is reached, and a lowering thrust force of the top ring shaft is measured by the load cell to determine the relationship between the torque limit value and the thrust force. Specifically, if the measured value of the thrust force is larger than the reference (design) thrust force (value having a certain range), the torque limit value is made smaller, and the same measurement is performed again. Conversely, if the measured value of the thrust force is smaller than the reference (design) thrust force (value having a certain range), the torque limit value is made larger, and the same measurement is repeated, whereby the torque limit value which falls within the range of the reference (design) thrust force (value having a certain range) is determined (searched). The torque limit value thus determined is set as a torque limit value for the pad search.
The inventors of the present invention have conducted various experiments and obtained the following knowledge. Specifically, in the polishing apparatus described above, a loss torque of the vertical movement mechanism for vertically moving the top ring is decreased by operation history, compared to startup operation. Because the limit value is set for the motor torque of the vertical movement mechanism to determine the position at which the top ring is brought into contact with the surface of the polishing pad, the change of the loss torque of the vertical movement mechanism has an effect on a thrust force for pressing the top ring against the surface of the polishing pad. When the loss torque of the vertical movement mechanism is reduced, the thrust force for pressing the top ring against the surface of the polishing pad is increased by an amount corresponding to the reduction, thus pressing the top ring against the polishing pad with more powerful force. Consequently, if the pad search is conducted at a predetermined period determined by the number of processed substrates, the wear amount of a retainer ring, or the like, the top ring is pressed excessively. Therefore, a gap between the surface of the polishing pad and the elastic membrane provided in the top ring for pressing the substrate cannot be kept constant. As a result, the polishing process condition varies, causing adverse effect on a process performance, such as poor uniformity of the surface, being polished, of the substrate.